Surface discharge spark plug



May 27, 1958 w. B. sMTs SURFACE DISCHARGE SPARK PLUG Filed Dec. 30, 1953United States Patent Ohce 2,836,756 Patented May 27, 1958 SURFACEDISCHARGE SPARK PLUG Wytze Beye Smits, Voorburg, Netherlands ApplicationDecember 30, 1953, Serial No. 401,271

3 Claims. (Cl. 313-131) The invention relates to a surface dischargespark plug for low voltages, for example, below about 4,000 volts, inwhich the Separator body located between the electrodes consists of aglass or enamel mass.

Spark plugs of this kind are already known, in which the space betweenthe electrodes is filled up completely with a mass of molten glass orglassy material. The object of such spark plugs is to prevent theappearance of cracks in the glass Separator body. lt has been found thatupon the glass mass between the electrodes gradually wearing away duringoperation the sparking path of the spark becomes so great that in thecourse of time the voltage of the electric energy to be supplied to thespark plug becomes too high.

The invention has for its object to furnsh an improvement, and thisobject is attained in that below the glass mass a body of insulatingmaterial which is resistant to great heat, for example, ceramicmaterial, has been provided, which penetrates into the glass massbetween the electrodes. Preferably this part is shaped like a cone, andthe apex of the cone penetrates a short distance into the glassymaterial between the electrodes.

This has the result that the spark can never burn in deeply and alwaysemerges above the electrodes, and that the path along which the surfacedischarge spark travels will always remain shorter than is the case in aspark plug in which the entire space between the electrodes is filled upwith a glass mass which gradually burns away completely, so that thespark can burn in deeply into this mass and is not arrested until thespark impinges on the ceramic body, while according to the inventon theapex of the cone of the ceramic body provides for the spark beingconstantly drected outwards, so that a strong spark is always producedin the combuston chamber of the engine.

The electrodes of the spark plug according to the invention arepreferably made of a material which is greatly resistant to hightemperatures, such as molybdenum or tungsten, the coefiicient ofexpansion of which is equal or practically equal to that of the holdersin which the electrodes are mounted, while the coefiicients of expansionof the intermediate glass mass or the like and the ceramic body have thesame value.

The inventon is elucidated further with reference to the drawingillustrating an embodiment.

Fig. 1 illustrates the spark plug according to the invendon.

Fig. 2. is a vertical cross-section through the sparking element.

Fig. 3 is a cross-section through part of the sparking element, shown ona greatly enlarged scale.

For a clear distinction between the known spark plugs and that accordingto the invention reference is made to Fig. 3.

In this figure, 1 is the holder of the outer electrode 2, and 3 theholder of the inner electrode 4, which electrodes consist, for example,of tungsten, preferably made from a drawn tungsten wire of circularcross-section, which has been polished. Between the electrodes 2 and 4there is disposed a mass of molten glass 5, while into this glasspenetrates a conically shaped ceramc body 6; the remaining space '7 isalso filled up with molten glass. The electrode surfaces have toconverge towards the sparking surface, so as to prevent the glassfalling out.

f the space between the electrodes 2 and 4 were to be filled upcompletely with glass, as is the case in some known spark plugs, andsmall cracks should arise in the glass owing to the varying temperaturesto which it is subjected, at the bottom of the electrodes an electricdsruptive discharge would take place, so that the spark plug wouldbecome worthless.

If further between the electrodes were to be disposed glass alone and aceramic body extending almost to the bottom of the electrodes, with thewearing away of the electrode material the requisite voltage willincrease disproportionately, because the creeping path extends to belowthe electrodes and the spark is not arrested until it impinges on theceramic body.

According to the invention these disadvantages are climinated, theceramic body 6 on the contrary being conical, and the apex of the conepenetrating into the glass mass 5 to about one-third of the height ofthe electrodes. It is found that when the spark impinges on the ceramicbody, with the gradual wearing away of the glass mass 5, the voltageneed be raised only slightly, because the length of the creeping pathincreases only slowly. When even the electrodes have worn away to thebottom, the length of the creeping path of the ceramic body is such thatit is bridged by the voltage value of the coil.

Since the electrodes 2 and 4 are made from a tungsten or molybdenumwire, the holders have to consist of a steel with the same coeficient ofexpansion; the coefiicients of expansion of the molten material and theceramic body likewise have to be of the same order of magnitude. Sincethis molybdenum or tungsten adheres only to glass or enamels which aremelted between the electrodes, they are covered with a thin galvanizedcoating of copper, nickel or the like, which guarantees good adhesion tothe molten material.

I claim:

1. A low tension surface discharge spark plug comprising, twoelectrodes, a mass of glassy material separating said electrodes andforming at the surface thereof a path over which a spark travels, and aheat resistant insulating material penetrating said glassy mass from thelocation remote from said surface and having at least a portion thereoflying between said electrodes.

2. A spark plug according to claim 1 in which said insulating materialtapers to a point toward said surface, said point lying a distance fromsaid surface which is at least two-thirds the depth of said electrodes.

3. A spark plug according to claim 1 in which said heat resstantinsulatve material is ceramic.

References cited in the file of this patent UNITED STATES PATENTS2,122,57l Hastings July 5, 1938 2,235,325 Mohrle Mar. 18, 1941 2,527,489Smits Oct. 24, 1950

